Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized further issues as a result of operation with such systems. As one example, an exhaust passage may be coupled to the second exhaust manifold and include the turbine. First and second emission control devices may be positioned in the exhaust passage, downstream of the turbine. At high engine power levels, the engine may be over fueled (e.g., run rich of stoichiometry) to reduce exhaust valve and turbine temperatures. However, this may cause high levels of hydrocarbons and carbon monoxide in the exhaust passage. When the second emission control device is a three-way catalyst, non-stoichiometric exhaust gases entering the second emission control device may result in reduced three-way catalyst function and thus, increased emissions. When the second emission control device is a gasoline particulate filter, the increased levels of hydrocarbons may result in increased soot loading of the GPF and thus, increased emissions.
In one example, the issues described above may be addressed by an engine operating method, comprising: flowing air from an intake manifold through a plurality of engine cylinders to a junction of an exhaust passage and a bypass passage in response to a condition, the junction positioned along the exhaust passage between first and second emission control devices; and flowing exhaust gas to the first emission control device while flowing the air to the junction. The air may be blowthrough air that has not undergone combustion. As one example, the condition may be a request to regenerate the second emission control device. In another example, the condition may be a flow rate of gases entering the engine and/or a power level of the engine over a threshold power level. In this way, by flowing air to the second emission control device, increased oxygen may enter the second emission control device.
In one example, this increased oxygen may help to maintain a stoichiometric mixture entering the second emission control device, thereby increasing function of the second emission control device and reducing engine emissions. In another example, this increased oxygen may help to regenerate and burn soot from the second emission control device and thus also result in increased function of the second emission control device and reduced emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.